1. Field of the Invention
The invention relates to a welding auxiliary material for welding refractory metal parts of high-intensity electric light sources.
2. Description of the Prior Art
When manufacturing halogen lamps, the joints between the refractory metal parts such as tungsten or molybdenum are welded joints, as the reliability of the joints is an indispensable requirement.
Refractory metal parts of low-wattage halogen lamps with "standard", i.e. not long or extended life, are welded directly together, if no special requirement exists. This, however, does not ensure a satisfactory joint in every case. The Hungarian Patent No. 185.198 discloses a molybdenum lead-in foil for elimination of unsatisfactory features. The surface of the foil is provided with a thin electroplated rhenium layer. Such a lead-in foil has better weldability properties than the simple uncoated foil. A foil prepared in that way is useful in the manufacture of halogen lamps with low current loads and "standard" lives, for joining refractory metal parts and meets the requirements for reliable operation on that field, but not in the case of halogen lamps with extended or long lives and heavy current loads.
This is explained by the fact that only a low-thickness rhenium layer can be applied to the surface of the lead-in foil. This has the consequence that, during the welding operation, the lead-in wire is unable to get embedded into the thin rhenium layer applied to the lead-in foil and therefore, a small-surface weld will be produced. In case of high-wattage halogen lamps, the current flow - due to the high current density-generates a substantial amount of thermal load resulting in the earlier destruction of the joint and in shortened lamp life.
The functionability of halogen lamps with high current loads and lives of several thousand hours can only be insured if higher requirements for joining the refractory metal parts and for the bond produced in welding are met.
In some types of lamps with high current loads as well as long lives, a separate platinum welding auxiliary is placed between the basic metal foil and the current lead-in order to produce a welded joint of satisfactory quality.
In this case, platinum will melt during welding and form a molybdenum-platinum alloy phase with a portion of the molybdenum material of the foil. This adversely affects the strength at high temperatures of the molybdenum foil which can result in thickness decrease, cracks and tear in the foil.
Another disadvantage is that after melting at the temperature necessary for pinch-sealing the lamp liquid and/or vapour phase platinum can enter the inner space of the lamp where, on reaching the tungsten filament, a lower-melting platinum-tungsten alloy will be formed. This has the possible consequences of local filament fusing and arcing that result in early lamp failure.
It should also be considered a disadvantage that the melt creeps on the current lead-in during welding with the consequence that no gas-tight seal will be achieved in some cases.
A further disadvantage is that the so-called "brazed joint" will in lamp operation, caused by the local thermal load, get alloyed gradually into the basic metals during the time of operation. Caused by this "loss" of the initial bond, the cross-section available for current conduction will decrease and the local thermal load will be enchanced resulting in a further "loss" and a rapid deterioration of the joint. It is also a common practice to use a platinum coated welding auxiliary material for welding refractory metal parts together. This method suffers from the disadvantages described earlier in the discussion of platinum welding auxiliary material.
According to the U.S. Pat. No. 4,823,048, a joint is produced by spot-welding the inner current lead-in and the metal foil together and a welding auxiliary material foil is connected, also by means of welding, to this joint. The filler foil increases the surface area available for current conduction thereby reduces current density. One end of the metal foil is connected to the large-surface portion of the metal foil opposite to the inner current lead-in and its other end, to the inner current lead-in.
The joint according to U.S. Pat. No. 4,823,048 has the disadvantages of being complicated to produce and also the inability to solve the problems described earlier related to the cross-section reduction or tear of the current lead-in foil.